Coating device and method



Sept. 27, 1955 P. J. CLOUGH ET AL 2,719,094

COATING DEVICE AND 'METHOD Filed June 16, 1951 Power pp y FIG. 3

IN V EN TORS James H Moore L MA Ln) W ATTORNEY United States. PatentCOATING DEVICE AND METHOD Philip J. Clough, Reading, and James H. Moore,Swampscott, Mass, assignors to National Research Corporation, Cambridge,Mass., a corporation of Massachusetts Application June 16, 1951, SerialNo. 231,916

6 Claims. (Cl. 117--'-22) This invention relates to coating and moreparticularly to the coating of substrates by vacuum evaporation ofaluminum with deposition of the aluminum vapors on the substrate. Thisinvention is particularly directed to improvements in sources foraluminum vapors in such vacuum-coating processes.

It is a principal object of the present invention to provide an improvedsource of aluminum vapors for use in a vacuum coating process.

Another object of the invention is to provide such a source which ischeap, reliable, and capable of long periods of operation with thegeneration of large quantities of aluminum vapors.

Another object of the invention is to provide a support for moltenaluminum during the evaporation thereof, this support being readilywettable by molten aluminum and being relatively inert to moltenaluminum.

Still another object of the invention is to provide an improved methodfor manufacturing such supports.

Still another object of the invention is to provide improved coatingprocesses and apparatus embodying such improved aluminum vapor sources.

Other objects of the invention will in part be obvious and will in partappear hereinafter. V

The invention accordingly comprises the process involving the severalsteps and the relation and the order of one or more of such steps withrespect to each of the others, and the apparatus possessing theconstruction, combination of elements and arrangement of parts which areexemplified in the following detailed disclosure, and the scope of theapplication of which will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconnection with the accompanying drawings wherein:

Fig. 1 is a diagrammatic schematic sectional view of one form of coatingapparatus utilizing the present in vention.

Fig. la is an enlarged view of a portion of Fig. 1.

Fig. 2 is a diagrammatic schematic sectional view of an apparatus usefulin preparing the novel aluminum supports of the present invention.

Fig. 3 is a schematic top view of one type of aluminum support elementparticularly useful for batch coating operations.

In one preferred embodiment of the invention there is provided a vacuumchamber in which a substrate to be coated may be moved past a source ofmetal vapors, these metal vapors being preferably aluminum, and theinvention being primarily described in connection with the coating ofaluminum on flexible and other substrates. One preferred type ofaluminum vapor source comprises an elongated element which acts as asupport for aluminum, this element having a surface which is readily wetby molten aluminum and which is relatively inert 'to molten aluminum.For reasons of economy and because of its structural strength at hightemperatures, .this elon- 5 1 2,719,094 Patented Sept. '27, 1955 gatedsupport preferably comprises carbon, the surface of this carbon elementhaving a coating of one of the metals tungsten or molybdenum. This metalcoating preferably includes a substantial amount of a correspondingcarbide, these two metals and their carbides being readily wet by moltenaluminum and their carbides being relatively inert to molten aluminum.In the process of practicing the present invention some of the metal, atleast at the beginning of the use of the carbon element, may not becompletely converted to a carbide. At the temperatures preferablyemployed for vaporizing aluminum, i. e., 1300 to 1500 C., bothmolybdenum and tungsten have extremely low vapor pressures.

In a preferred method for manufacturing such an elongated carbon elementhaving a surface stratum of a carbide of one of the elements, molybdenumand tungsten, a coating of the corresponding metal is first created onthe surface of the carbon rod. Thereafter the carbon rod with themetallic coating is heated in an air-free atmosphere fora sufficientlength of time to convert a substantial proportion of the metal coatingto a corresponding carbide. This heating may be achieved in an inertatmosphere, such as argon, but it is preferred that this heating beaccomplished under a vacuum, since it simultaneously serves to drivefrom the carbon element those materials which might subsequently bevolatilized when the rod is heated to the relatively high temperaturesrequired for vaporizing aluminum in the vacuum-coating system. Thisheating process may thus serve the dual function of converting themetallic coating to a corresponding metal carbide and also of outgassingthe carbon element so that it will not interfere with the subsequentaluminum coating process.

In a preferred embodiment of this aspect of the invention the metalcoating is put on the carbon element by contacting the surface of thecarbon element with a finely divided solid carbonyl of the metal to beapplied to the carbon element. The interface between the carbon elementand the carbonyl is then heated to a temperature on the order of 300 C.for a sufficient time to decompose the carbonyl and to form a denseadherent coating of the metal on the carbon element. The surface of theelement is then freed of the carbonyl and is heated, as mentionedpreviously, to convert this metallic coating to the correspondingcarbide.

Referring now to Figs. 1 and 1a there is shown one preferred embodimentof the invention as applied to a continuous coating apparatus. Thisapparatus generally includes a vacuum-tight housing 10 definingtherewithin a vacuum-coating chamber 12 which is arranged to beevacuated by a vacuum-pumping system not shown. Within this chamber thesubstrate 16 to be coated is guided from a supply 17 thereof past aplurality of guiding rolls 18 to a take-up spool 19. If desired, therolls 18 may be cooled by suitable means. During the passage of thesubstrate from the supply 17 to the take-up spool 19, it passes in aseries of convolutions near a source 20 of metal vapors, these vaporsbeing preferably aluminum vapors. The guiding rolls 18 are preferablydriven and are so arranged that they do not see the vapor source. Thusthe guiding rolls are not coated by the vapors from the source, and arenot exposed to direct heat radiation from the source. This arrangementof rolls permits sufficient cooling during and between aluminum coatingsteps and provides a minimum of outgassing of heat-sensitive substrate.The source 20 preferably comprises an elongated carbon rod having agroove 24 in the upper surface thereof, this groove being preferablymaintained full of molten aluminum 26. This carbon rod is preferablysupported in a manner similar to that illustrated in the copendingapplication of Clough and Godley, Serial No. l7l,432, filed June 30,1950.

ice

Aluminum in either liquid or solid form may be fed to the groove in therod, as pointed out more fully in the above-mentioned application.

In the operation of the device shown in Figs. 1 and la, roll 17 of thesubstrate 16 is positioned within the vacuum chamber 12 and thesubstrate is guided around the various rolls 18 and connected to thetake-up spool 19. A coolant, such as cooling water at about 40 F. orlower, is then circulated through the rolls 18 by suitable piping (notshown) so as to chill the rolls 18. The vacuum chamber 12 is thenevacuated to a low pressure on the order of less than one micron, bymeans of the vacuum pumping system. When the requisite low pressure isachieved the substrate is advanced, preferably by driving the take-upspool 19 and the various guiding rolls 18. Aluminum is fed to the slot24 in the carbon rod 22 and both the rod and the aluminum carriedthereby are heated to a high temperature, on the order of between 1300and 1500 C., by passing a high amperage electric current through the rodand the aluminum supported thereby. During evaporation, the moltenaluminum wets a substantial portion of the rod surface outside of thegroove so that aluminum is evaporated from all hot surfaces of the rodthat see the substrate. This wetting by the molten aluminum is shown at26a in Fig. 1a.

In a preferred embodiment of the invention described above, the rod 22preferably comprises dense carbon, several preferred grades of densecarbon rod being those manufactured by Becker Bros. under thedesignation B-4, by National Carbon under the designation G-A, and byStackpole under the designation ClH. These carbon rods may have adiameter of about .50 inch and the groove 24 may be approximately A inchwide by 7 16 inch deep. The coating on the carbon rods comprisesmolybdenum or tungsten carbide which has been prepared in accordancewith the techniques described more fully hereinafter.

Referring now to Fig. 2 there is shown one preferred method ofmanufacturing a carbon rod having a carbide surface of the typementioned above. In this figure, 30 represents a container such as aglass beaker which confines a quantity of powdered carbonyl 32, thiscontainer being closed by a stopper 35. The carbon element 22 is heatedby a high frequency induction coil 36 connected to a suitable powersupply 37. If desired a thermocouple 38 may be utilized for indicating,by means of a meter 39, the temperature of the carbon element during thetreatment thereof. The container 30 is preferably vented through a tube40 containing a quantity of copper oxide 41. The tube also includes acalcium chloride water vapor barrier 42, a pressure surge tank 46, andanother container 48, the end of tube 40 being below the level of awater solution of potassium hydroxide 47 held in container 48.

In one preferred example of this aspect of the process of the presentinvention, the powdered carbonyl 32 is prepared by powdering molybdenumhexacarbonyl, M(Co)e. The crusted grains of carbonyl powder arepreferably of a size at least as small as 60 mesh, this small grain sizebeing particularly desirable for assuring uniform coating in the slot 24in the carbon rod 22. Powdered carbonyl 32 is then placed in a containerand the carbon rod 22 is inserted therein. In this connection it isdesired to tap the container 30 so as to insure intimate contact betweenthe powdered carbonyl and all surfaces of the rod. The high frequencycoil is then energized to heat the carbon rod to a temperature above thedecomposition temperature of the carbonyl. This temperature ispreferably on the order of approximately 300 C. This temperature ispreferably maintained for about 10 seconds, the temperature beinglowered to about 150 C. at the end of the 10 seconds and then againraised to 300 C. This cycle is repeated until the total time of heatingto a temperature near 300 C. equals about 180 seconds. During theoperation of the carbonyl decomposition apparatus the copper oxide 41 ispreferably heated to about 320 C. to convert to carbon dioxide thecarbon monoxide resulting from decomposition of the carbonyl. Oneconvenient method of operating the apparatus of Fig. 2, without the useof a thermocouple for directly reading the rod temperature, is toreplace the hydroxide solution 47 with water. Coil 36 is then energizeduntil a rapid series of bubbles appears in the water 47. This indicatesthat decomposition of the carbonyl is taking place, the bubbles beingcarbon dioxide. The coil is deenergized and the carbon rod 22 is allowedto cool slightly for a few seconds. The coil is energized again untilanother group of bubbles appears in water 47. Again the power is turnedoff. This cycle can be repeated until the total time for generating thebubbles equals about 180 seconds. At the end of either of the aboveheating periods the coil 16 is deenergized and the carbon rod isremoved.

In a preferred embodiment of the invention, particularly whenmanufacturing support elements for batch coating operations, the carbonrod may be, in the form illustrated at 50 in Fig. 3, abouttwo incheslong, about A inches in diameter. This rod has a slot 52 thereinapproximately l%" long. This carbon rod is preferably formed of a densecarbon, such as Grade B4 sold by Becker Bros. The slot 52 in the rod maybe about inch wide by & inch deep. A supply of aluminum in the slot 52is shown in the form of balls 54. The ends 56 of the rod are preferablytapered to provide low resistance contacts with a rod holder, not shown,to permit the rod to be heated by passing an electrical currenttherethrough. After coating such a rod, by heating in intimate contactwith powdered molybdenum hexacarbonyl at a temperature of about 300 forapproximately 180 seconds, the rod is found to be coated with a denseuniform layer of molybdenum which appears to be slightly less than .001inch thick. This coating is strongly adherent to the carbon element.Metallographic examination indicates that the molybdenum coating is inthe form of a laminar deposit. This coating is essentially molybdenumwith some carbon (less than 5%) in solution. This coated carbon element,after removal from the carbonyl coating container, is heated in a vacuumfurnace to approximately 1250 C. until the pressure in the vacuumchamber can be maintained at about .1 micron Hg abs. Metallographicexamination after treatment indicates that the molybdenum coating hasundergone recrystallization with considerable diffusion of carbon intothe molybdenum coating, particularly at the interface between themolybdenum coating and the carbon element. This difiusion of carbonappears to create an interface layer of molybdenum carbide, butmetallographic examinations alone cannot confirm this fact. A rod 50,prepared as above, was placed in a batch vacuum coater and the slot 52was loaded with about .33 gram of aluminum. Prior to the firstevaporation the temperature of the rod was gradually raised to slightlybelow the melting point of aluminum during the evacuation of the vacuumchamber to remove any adsorbed water vapor from the rod. During thispreliminary heating the pressure was maintained below about 5 microns Hgabs. When the operating pressure of approximately .1 micron Hg abs. wasobtained in the vacuum coater, the rod containing the aluminum washeated to about 1400 C. to evaporate all of the aluminums. This heatingwas preferably achieved by passing about 200 amps through the rod. Onsubsequent coatings the rod, reloaded with aluminum, is brought up totemperature (about 1400 C.) as soon as the desired coating pressure ofabout .1 micron has been obtained. This coating procedure was repeatedfor fifty times, approximately .33 gram of aluminum being evaporated oneach coating cycle. At the end of fifty batch coating operations the rodwas still in excellent shape.

From the above description of the invention it would:

seem that this carbon element containing the molybdenum-carbon surfacestratum is extremely resistant to attack by molten aluminum atrelatively high temperatures onthe order of 1300 to 1500 C., and has anamazingly long useful life. This is particularly surprising when it isconsidered that a molybdenum filament, when used in a batch coatingoperation for evaporating aluminum, has a life of about threeevaporations. An uncoated carbon rod has an equally short life and isvery poorly wet by molten aluminum. The exact mechanism involved is notparticularly well understood since the amount of molybdenum is extremelysmall. It is believed that, although there may be some slight solubilityof the molybdenum or molybdenum carbide in the molten aluminum, thedissolved molybdenum or molydenum carbide is redeposited as the aluminumevaporates, thus maintaining a protective surface stratum as long as therod is in contact with the molten aluminum.

The results utilizing tungsten carbonyl in place of molybdenum carbonylare equivalent to those obtained when using molybdenum carbonyl. Equallya carbonyl powder diluted by an inert powder may be employed in thiscoating operation, the other conditions, such as temperature, etc.,remaining the same. One type of diluent may comprise alumina powder ofabout 60 mesh or finer, the diluent being about 50% by weight of themixed powders. Other diluents are described in the copending applicationof Stauifer S. N. 181,373 filed August 25, 1950.

While a preferred embodiment of the invention has been described above,where molybdenum and tungsten carbonyls have been employed in a packtechnique utilizing powdered carbonyls, other less preferred embodimentsof the invention may be practiced. For example, the carbonyl may bedecomposed from the vapor phase, as described in British Patents 589,966and 589,977. Additionally, the molybdenum or tungsten coating can beproduced on the carbon element by thermal decomposition or reduction ofvolatile halides of these compounds. Such techniques are described inJournal Electro-Chemical Society 96, 318-333 (1949); van Arkel, ReineMetalle published by Edward Bros, Inc., Ann Arbor, Michigan 1943)Childs, Cline, Risner and Wulfi, Trans. A. S. M. 43, 105121 (1951).

These latter methods of coating the molybdenum or tungsten on the carbonelement are less preferred than the powdered carbonyl technique in viewof the simplicity and excellent results obtained by utilizing thispowdered carbonyl.

In general, it is desirable that the carbon element be as dense aspossible, and carbon or mixtures of carbon and graphite are preferredover dense graphite in .view of the higher resistance of these materialsto attack by molten aluminum in the event of imperfections in thecoatings. Graphite also has a low electrical resistance and, for a givenrod diameter, much higher electrical heating currents are required.Additionally, the rods may be formed of tungsten or molybdenum, and thesurfaces thereof may be converted to the carbide by heating in contactwith powdered carbon or graphite, or by thermal decomposition of simpleorganic compounds, such as methane. However, this alternative method ofmanufacturing these support elements is less preferred due to the lowerelectrical resistance of the resulting elements and the brittleness ofthe molybdenum or tungsten rods after use due to recrystallizationthereof at the high temperatures required for vaporizing aluminum. Alsothe carbon, in the surface carbide layer, will diffuse into themolybdenum or tungsten rod during use thereof, thus adding to thebrittleness of the rod. When the word carbon is used in the annexedclaims, it is intended to include elemental carbon, graphite andmixtures of carbon and graphite.

Since certain changes may be made in the above process and apparatuswithout departing from the scope of the invention herein involved, it isintended that all matter contained in the above description, or shown inthe accompanying drawings, shall be interpreted as illustrative and notin a limiting sense.

What is claimed is:

1. The process of manufacturing a wick element which is readily wet bymolten aluminum for use as a support for molten aluminum during thevacuum evaporation of aluminum in a vapor deposition coating process,said manufacturing process comprising the steps of providing anelongated support comprising a dense carbon element, contacting thesurface of said support with a finely divided solid carbonyl from theclass consisting of the carbonyls of the metals molybdenum and tungsten,maintaining the interface between said support and said carbonyl at atemperature on the order of 300 C. for a suflicient time to decomposesaid carbonyl and to form a dense adherent coating of said metal on saidsupport, freeing said surface of said carbonyl, and heating said supportunder a vacuum of at least .001 mm. Hg abs. for a sufficient time tooutgas said support and to cause at least some diffusion of carbon intosaid metal coating, said last named heating step being accomplished at atemperature on the order of at least 1000 C.

2. The process of manufacturing a wick element which is readily wet bymolten aluminum for use as a support for molten aluminum during thevacuum evaporation of aluminum in a vapor deposition coating process,said manufacturing process comprising the steps of providing anelongated support comprising a carbon element, applying to the surfaceof said carbon element a dense adherent coating of a metal taken fromthe group consisting of molybdenum and tungsten, said coating having athickness between about .00004 inch and .001 inch, and heating saidsupport under a vacuum of at least .001 mm. Hg abs. for a sufiicienttime to outgas said support and convert at least some of said metalcoating to a corresponding metal carbide, said last-named heating stepbeing accomplished at a temperature on the order of at least 1000" C.

3. The process of manufacturing a wick element which is readily wet bymolten aluminum for use as a support for molten aluminum during thevacuum evaporation of aluminum in a vapor deposition coating process,said manufacturing process comprising the steps of providing a carbonsupport element, contacting the surface of said support with a finelydivided solid carbonyl from the class consisting of the carbonyls of themetals molybdenum and tungsten, maintaining the interface between saidsupport and said carbonyl at a temperature on the order of 300 C. for asufiicient time to decompose said carbonyl and to form a dense adherentcoating of said metal on said support, freeing said surface of saidcarbonyl, and heating said carbon element under a vacuum at some stagein its manufacture to a temperature of at least 1000 C. untilsubstantially all volatiles, having a vapor pressure greater than about0.5 micron Hg abs. at 1000 C., have been removed from said element.

4. The process of manufacturing a wick element which is readily wet bymolten aluminum for use as a support for molten aluminum during thevacuum evaporation of aluminum in a vapor deposition coating process,said manufacturing process comprising the steps of providing anelongated support comprising a carbon element, depositing on said carbonelement a dense adherent coating comprising a metal from the classconsisting of the metals molybdenum and tungsten, heating said coatedelement to a temperature of at least 1000 C. in an air-free atmosphereto cause a reaction between said carbon and said metal coating, andheating said carbon element under a vacuum at some stage in itsmanufacture to a temperature of at least 1000 C. until substantially allvolatiles, having a vapor pressure greater than about 0.5 micron Hg abs.at 1000 C., have been removed from said element.

5. A coating device for coating substrates with aluminum by evaporationof said aluminum and condensation of said aluminum on said substrate,said apparatus comprising means defining a vacuum-tight housing, meansfor evacuating said housing to a free air pressure on the order of lessthan one micron Hg abs., an elongated support for molten aluminum Withinsaid housing, a charge of aluminum carried by said support, said supportcomprising a carbon element, said carbon element having thereon a denseadherent coating comprising a metal from the class consisting of themetals molybdenum and tungsten, at least a substantial portion of saidmetal coating on said carbon element having been converted to a carbideof said metal, and means for heating said carbon element, and thealuminum supported thereby, to a temperature on the order of above 1300"C.

6. A support for holding molten aluminum during evaporation thereof in avacuum coating device, said support comprising an elongated carbonelement, said carbon element having thereon a dense adherent coatingcomprisingat least one metal from the class consisting of the metalsmolybdenum and tungsten, said element being substantially free of allvolatiles having a vapor pressure greater than about 0.5 micron Hg abs.at 1000 C., said element indicating, under metallographic examination,considerable difiusion of carbon into said metal coating at theinterface between said metal coating and said carbon element.

References Cited in the file of this patent UNITED STATES PATENTS1,047,541 Lederer Dec. 17, 1912 1,741,477 Pfiffner Dec. 31, 19292,257,668 Becker et a1. Sept. 30, 1941 2,282,098 Taylor May 5, 19422,344,138 Drummond Mar. 14, 1944 2,363,781 Ferguson Nov. 28, 19442,516,058 Lander July 18, 1950 2,548,897 Kroll Apr. 17, 1951 2,604,395Gonser et al. July 22, 1952

1. THE PROCESS OF MANUFACTURING A WICK ELEMENT WHICH IS READILY WET BYMOLTEN ALUMINUM FOR USE AS A SUPPORT FOR MOLTEN ALUMINUM DURING THEVACUUM EVAPORATION OF ALUMINUM IN A VAPOR DEPOSITION COATING PROCESS,SAID MANUFACTURING PROCESS COMPRISING THE STEPS OF PROVIDING ANELONGATED SUPPORT COMPRISING A DENSE CARBON ELEMENT, CONTACTING THESURFACE OF SAID SUPPORT WITH A FINELY DIVIDED SOLID CARBONYL FROM THECLASS CONSISTING OF THE CARBONYLS OF THE METALS MOLYBDENUM AND TUNGSTEN,MAINTAINING THE INTERFACE BETWEEN SAID SUPPORT AND SAID CAR-